In sandy sediments, the burrow ventilation activity of benthic macrofauna can generate substantial advective flows within the sediment surrounding their burrows. Here we investigated the effects of such advective bio-irrigation on carbon and nitrogen cycling in sandy sediments. To this end, we combined a range of complementary experimental and modelling approaches in a microcosm study of the lugworm Arenicola marina (Polychaeta: Annelida). Bio-irrigation rates were determined using uranine as a tracer, while benthic fluxes of oxygen (O-2), total carbon dioxide (TCO2), dissolved inorganic nitrogen (NH4+, Sigma NO2-+NO3-) and dinitrogen (N-2) were measured in closed-core incubations containing lugworms acclimatized for a relatively short (2 d) and long (3 wk) duration. The fluxes induced by A. marina were compared to those induced by mechanical mimics that simulate the flow pattern induced by the lugworm. These mechanical mimics proved a useful tool to simulate the effect of l! ugworm irrigation on sediment biogeochemistry. Subsequently, reactive transport model simulations were performed to check the consistency of the measured fluxes and rates, and to construct closed mass balances for sedimentary nitrogen. This reactive transport model Successfully captured the essential features of the nitrogen cycling within the sediment. Advective irrigation by both lugworm and mechanical mimics significantly stimulated the sediments O-2 consumption, organic matter mineralization rate (TCO2 release), and denitrification rate (N-2 production). While sedimentary O-2 consumption was directly correlated to advective input of O-2, increasing irrigation rates increased the importance of coupled nitrification-denitrification over the external input of nitrate from the overlying water.